Mixed-signal PCB layout and wiring techniques will be illustrated through the design of the OC48 interface card. OC48 stands for Optical Carrier Standard 48, which is basically oriented to 2.5GB serial optical communication. It is one of the high-capacity optical communication standards in modern communication equipment. The OC48 interface card contains several typical mixed-signal PCB layouts and routing problems, and the layout and routing procedures will specify the sequence and steps to solve the mixed-signal PCB layout solution.
The OC48 card contains an optical transceiver for the bidirectional conversion of optical signals and analog electrical signals. The analog signals are input or output to the digital signal processor, and the DSP converts these analog signals into digital logic levels, which can be connected to the microprocessor, the programmable gate array, and the system interface circuit of the DSP and microprocessor on the OC48 card. Separate phase-locked loops, power filters, and local reference voltage sources are also integrated.
“Among them, the microprocessor is a multi-power device; The main power supply is 2V, 3.3V I/O signal power supply” is shared by other digital devices on the board. The standalone digital clock source provides the clock for OC48I/O, microprocessor, and system I/O.
After checking the layout and wiring requirements of different functional circuit blocks, 12-layer boards are initially recommended. Microstrip and stripline layers are configured to safely reduce coupling between adjacent routing layers and improve impedance control. A junction layer is set between the first and second layers to isolate the sensitive analog reference source, CPU core, and PLL filter power wiring from the microprocessor and DSP devices in the first layer. The power supply and connection layer always come in pairs, as is done for sharing the 3.3V power layer on the OC48 card. This will reduce the impedance between the power supply and the ground, thus reducing the noise on the power signal.
Avoid walking the digital clock lines and high-frequency analog lines near the power layer, otherwise, the noise from the power signal will be coupled to the sensitive analog signal.
Depending on the needs of digital signal wiring, careful consideration should be given to the use of power and analog ground splits, especially at the input and output ends of mixed-signal devices. Crossing an opening in the adjacent signal layer can result in impedance discontinuity and poor transmission line loops. These can cause signal quality, timing, and EMI issues.
Sometimes adding a number of connections, or using a number of outer layers for a local power layer or connection layer under a device, can eliminate openings and avoid this problem, as is the case with multiple connections on an OC48 interface card. Maintaining the laminated symmetry of the position of the opening layer and the wiring layer can avoid the distortion of the cards and simplify the fabrication process. Due to the high current resistance of the 1-ounce copper cladding plate, the 3.3V power supply layer and the corresponding grounding layer should be 1-ounce copper cladding plate, and the other layer can be 0.5 ounce copper cladding plate, which can reduce the voltage fluctuation caused by high transient current or peak period.
If you’re designing a complex system from the ground up, use 0.093 “and 0.100” thickness cards to support the wiring layer and the grounding isolation layer. The thickness of the card must also be adjusted according to the size of the through-hole pads and hole wiring characteristics so that the width to height ratio of the bore diameter to the thickness of the finished card does not exceed the width to height ratio of the metalized hole provided by the manufacturer.
If you want to design a low-cost, high-volume commercial product with the minimum number of wiring layers, carefully consider the wiring details of all the special power supplies on a mixed-signal PCB before laying out or wiring. Have the target manufacturer review the initial layering plan before starting layout and wiring. Basically, stratification should be based on the thickness of the finished product, the number of layers, the weight of copper, impedance (with tolerance), and minimum through pad and hole size. The manufacturer should provide recommendations for stratification in writing.
Examples of configurations for all controlled impedance stripe lines and microstrip lines should be included in the recommendation. Consider your impedance prediction in conjunction with the manufacturer’s impedance prediction. These impedance predictions can then be used to verify signal routing characteristics in simulation tools used to develop CAD routing rules.
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